Thermal ink-jet printers have become widely popular as inexpensive printing devices. An essential feature of a thermal ink-jet printer is a print head that is controlled to selectively eject tiny droplets of ink onto a printing surface, such as a piece of paper, to form desired images and characters.
The print head generally has an architecture plate with multiple tiny nozzles through which ink droplets are ejected. Adjacent to the nozzles are ink chambers, where ink is stored prior to ejection through the nozzles. Ink is delivered to the ink chambers through ink channels that are in fluid communication with an ink supply.
The print head usually is formed of a sandwich construction, having a substrate at its base. Attached to the substrate is a layer of circuit traces and a layer of the resistors. The resistors are overlaid with a protective, passivation layer. The architecture plate is bonded to the substrate and substantially covers the other layers.
The resistors are lined up beneath the chambers in the architecture plate. Electrical signal inputs to the resistors "fire" the resistors, heating the resistors and thereby a volume of ink within the adjacent ink chamber. The heating generates a vapor bubble in the ink to force an ink droplet out of the nozzle.
Usually, remote bus lines provide signal inputs from an external signal source to the resistors on the print head. Oftentimes, the signals are delivered through multiplexed circuitry on the substrate. The print head is generally connected to these bus lines by a thin flat electrical cable, such as a tape automated bond ("TAB") circuit. A TAB circuit generally has copper leads supported on a copper-coated tape. The tape is usually bonded onto the print heads with gold bump contacts. Conventional TAB circuit bonding cannot be done over live silicon circuitry without damaging the circuitry and requires use of an encapsulant to protect the leads from the ink, which adds a process step and decreases the robustness of the bond. Nevertheless, TAB circuit bonding is generally used because it is space-efficient, allowing the contact to be made in a tiny area.
In most ink-jet printers, the print head is mounted on an ink pen that is mounted to a carriage that traverses the printing surface to move the print head back and forth over the printing surface. Thus, the print head can be made relatively small in comparison to the width of the printing surface because the ink pen traverses the width of the printing surface. However, it takes the carriage a certain amount of time to traverse the paper, which slows down the speed of printing.
One way to increase the printing speed is to increase the number of nozzles on the print head, which necessitates an increase in the size of the print head. However, increasing the size of the print head requires a larger architecture plate, and a large architecture plate increases the likelihood of failure of the bonding of the interface between the architecture plate and the substrate. One reason for such failure is that the materials for the substrate and the architecture plate usually have considerably different coefficients of thermal expansion. Thus, the sandwich construction may bow or delaminate after assembly as the print head is heated and cooled during operation.
Sometimes, components within ink-jet printers are attached together by flip-chip processing. Flip-chip processing is the term used to describe the method of attaching two parts, such as a die and a substrate, by providing both parts with solderable pads, depositing a solder ball on the solderable pad on the substrate, then placing the solderable pad of the die on top of the solder ball, and heating and pressing the die and substrate to form a solder joint. Often, the solder ball is formed by depositing solder paste on the solderable pad on the substrate and heating the paste and pad to reflow the solder paste into a solder ball.
Oftentimes, after the two parts are attached by flip chip processing, an underfill made of liquid epoxy will be shot between the parts and allowed to wick therebetween, in a process separate from flip-chip processing. The underfill layer comprising the cured epoxy fills gaps between the parts and relieves some of the stress on the solder joint.
This invention provides a direct drive, page-wide array thermal ink-jet print head. Page wide array printheads have been disclosed in U.S. patent application Ser. No. 08/551,266, filed on behalf of Paul H. McClelland et al. on Oct. 31, 1995, titled "Large Area InkJet Printhead" and U.S. patent application Ser. No. 08/550,698, filed on behalf of Paul H. McClelland et al. on Oct. 31, 1995, titled "Printhead With Puny Driven Ink Circulation". These applications are assigned to the assignee of the present invention. The print head of the present invention allows an entire line of characters to be printed nearly simultaneously, which increases printing speed. The embodiment of the present invention is produced using a substantially modified version of an assembly process known as "flip chip" processing.
A preferred embodiment of the invention includes a substrate having a circuit trace layer deposited thereon. The substrate also carries a layer comprising a row of resistors attached to a portion of the circuit trace layer and to the substrate. A driver chip is attached over another portion of the circuit trace layer. The driver chip receives signals from the central processing unit and sends signals to the resistors. The circuit trace layer includes circuit traces that connect the driver chip to the resistors for sending signals to "fire" the resistors.
As another aspect of this invention, the driver chip is attached to the substrate using a soldering and polymeric bonding technique. For this technique, multiple interconnect pads are deposited over the circuit trace layer on the substrate. The interconnect pads are positioned in rows and are spaced further apart than are the resistors. A polymeric underfill layer is deposited over the substrate and interconnect pads. Because the underfill layer is deposited before the soldering is complete, the layer is referred to as a pre-placed underfill layer. An opening is created at each interconnect pad to expose the interconnect pad and form a cavity that surrounds the pad. Solder paste is deposited in each opening and then is reflowed to create a solder ball on each interconnect pad. The chip is provided with multiple solderable pads that are spaced to align with the interconnect pads. Each solderable pad is positioned near a solder ball. Heat and pressure are applied to the solder balls, interconnect pads, and solderable pads to melt the solder to form a solder joint and to polymerically bond the chip to the substrate.
This soldering and bonding technique can be used instead of TAB circuit bonding because the relatively increased spacing between the interconnect pads (compared to the resistor spacing) provided by the present invention affords more room in which to make an electrical interconnect. This technique is quicker than applying an underfill after soldering and expected to be more robust than conventional TAB bonding in thermal ink-jet applications.
As yet another aspect of this invention, the opening in the underfill layer at each interconnect pad is sized to contain an amount of solder paste that, when reflowed, will form an appropriate size solder ball for attaching the chip thereon. Thus, the underfill layer also acts as an in-situ stencil for measuring the correct amount of solder paste, thereby eliminating the need for a separate measuring step.
Using a pre-placed underfill reduces the number of process steps required to produce an assembly because the underfill does not need to be applied separately. Another advantage is that it controls the collapse of the solder balls during melting so that the balls do not unduly spread out due to the weight of the chip or the pressure that may be applied to the chip. The underfill between the substrate and chip also serves to adhere the two parts together and to relieve stress on the solder joint between the two parts. Yet another advantage is that the underfill layer has a very short bonding time, especially in comparison to the cure time required of liquid epoxy.